Special thanks to Ms Carmen Urich for helping me obtain articles needed to update this publication.

The authors and editors of Medscape Reference gratefully acknowledge the contributions of the following previous authors, Brian P. Green, DO, MS, PA-C; Sean T. Gunning, MD; and Mary K. Mather, MD, to the development and writing of this article.

The authors and editors of Medscape Reference would also like to thank Wayne M. Meyers, MD, for his clinical images from his article with Douglas S. Walsh, MD, in Transactions of the Royal Society of Tropical Medicine and Hygiene.
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